1. Field of the Invention
The present invention relates to a circular accelerator that accelerates charged particles to high energy while making them orbit along a near-circular spiral orbit, and extracts the accelerated charged particles to the outside thereof.
2. Description of the Related Art
A synchrocyclotron and cyclotron are known as the device that accelerates charged particles to high energy while making them orbit along a spiral orbit. In order to stably accelerate the charged particles in those synchrocyclotron and cyclotron, the following are required: A predetermined radio-frequency acceleration electric field is applied in a beam-traveling direction in accordance with the timing of the particles crossing an acceleration electrode. Predetermined converging force is provided in the beam-traveling direction and also in a direction perpendicular to the beam.
In a synchrocyclotron as described in Patent Document 1, for example, charged particles produced in an ion source are gradually accelerated every time they cross its acceleration electrode while forming an orbit by a bending electromagnet. The radius of the orbit grows larger as their energy increases, that is, the orbit becomes a spiral, and when accelerated to reach their maximum energy, the charged particles are extracted from an extraction duct to the outside. The synchrocyclotron described in Patent Document 1 is configured in a way as follows:    (1) The resonant frequency of an acceleration electrode portion is modulated at high speed with a period of some 1 kHz during acceleration, so as to accelerate the particles by the radio-frequency acceleration electric field frequency-modulated at high speed.    (2) Converging force in a beam traveling direction is secured.    (3) Converging force in a direction perpendicular to the beam can be secured owing to the weak converging magnetic field.
High-speed modulation of the resonant frequency at a 1 kHz level is extremely difficult in the device described in Patent Document 1.
In a cyclotron as described in Patent Document 2, for example, charged particles generated in an ion source are gradually accelerated every time they cross its acceleration electrode while forming an orbit by a bending magnetic field generated by a bending electromagnet. The radius of the orbit grows larger as the charged particles are accelerated to increase their energy, that is, the orbit becomes a spiral orbit, and when the charged particles are accelerated to reach their maximum energy, they are extracted from an extraction duct to the outside. These operations are so far the same as those of the synchrocyclotron.
In order to stably accelerate the charged particles in the cyclotron,    (4) A predetermined radio-frequency acceleration electric field is applied in a beam traveling direction, at the timing of the charged particles crossing the acceleration electrode.    (5) Predetermined converging force is provided in a direction perpendicular to the beam.are required, and in addition,    (6) No converging force is provided in the beam traveling direction.
In the cyclotron described in Patent Document 2, regarding the above (4), since magnetic field distribution by the bending electromagnet is formed in such a way that the orbital frequency of the charged particles does not vary depending on acceleration, the frequency of the radio-frequency acceleration electric field does not need to be modulated. This magnetic field is referred to as an isochronous magnetic field. In terms of the above (6), since no converging force is provided in the beam traveling direction in the isochronous magnetic field, the cyclotron is configured in a way such that accuracy of shaping the magnetic field by the electromagnet is raised up to some 1×10−6, and in addition, the acceleration voltage is increased so as to extract the beam after turning several hundred times or so. Moreover in terms of (5), in order to generate the isochronous magnetic field, the magnetic field needs to become stronger as the radius increases, which causes large diverging force in the beam perpendicular direction. In order to overcome this diverging force and obtain converging force in the perpendicular direction, the bending electromagnet is configured with a large magnetic pole gap and a small magnetic pole gap repeated alternately in an orbiting direction of the charged particles, and in addition, magnetic poles are shaped in a spiral.    Patent Document 1: International Patent Publication WO2006/012467    Patent Document 2: International Patent Publication WO91/07864
Problems with the conventional circular accelerators have been as follows: Both the synchrocyclotron in Patent Document 1 and cyclotron in Patent Document 2 can hardly Vary acceleration energy by one accelerator to accelerate to a several hundred MeV level so that they can be used for particle beam therapy. Moreover, in the synchrocyclotron in Patent Document 1, high-speed modulation of the resonant frequency of the radio-frequency acceleration electrode portion is needed during acceleration, and since the portion to which high power is supplied is driven at as high speed as 1 kHz, it is difficult to secure reliability. On the other hand, in the cyclotron in Patent Document 2, required accuracy of the magnetic field by the electromagnet must be some ΔB/B=1×10−6, which therefore needs troublesome work such that magnetic field measurement and machining the magnetic poles are alternately repeated at the site where it is actually installed, so as to realize the foregoing accuracy.